This invention relates to a communication system which carries out code transform as forward transform on a sequence of input samples in an encoding device so as to enable high efficiency coding and which carries out inverse code transform in a decoding device.
A conventional communication system of the type described comprises an encoding device which is supplied as a sequence of input samples with a sequence of digital image signals and which subjects the digital image signals to code transform, such as DCT (discrete cosine transform) or the like, by the use of a code transform circuit to internally produce a sequence of code transformed signals. After the code transform, the code transformed signals are very often subjected to interframe predictive coding to produce a sequence of error signals and is thereafter sent to a decoding device as a sequence of output digital signals. More specifically, the encoding device comprises a subtracter supplied with the code transformed signals and a sequence of local decoded signals sent through a local decoding loop. The subtracter calculates differences between the code transformed signals and the local decoded signals and produces the error signals representative of the differences. The error signals are sent from the encoding device to the decoding device as the output digital signals on one hand and are locally decoded by the local decoding loop into the local decoded signals on the other hand. Within the local decoding loop, the error signals are subjected to inverse code transform by the use of an inverse code transform circuit and thereafter locally subjected to the code transform in a manner similar to that of the code converter.
Thus, the inverse code transform and the local code transform are always executed in the local decoding loop with a predetermined precision which is finite. In other words, transform errors inevitably occur during such inverse code transform and local code transform in dependency upon the precision and are successively accumulated with time.
It is to be noted that the code transform, such as the orthogonal transform, is usually carried out by dividing the input samples into sample blocks each of which is composed of at least one input sample. In addition, the input samples or the digital image signals carry or convey not only a moving image but also a stationary image. This shows that the sample blocks can be classified into a moving image block and a stationary image block.
When the stationary image block is subjected to the interframe predictive coding, only less significant coded data signals have to be used during the interframe predictive coding and may not be sent from the encoding device to a decoding device. In other words, significant coded data signals have to appear during the interframe predictive coding of only the moving image block. Thus, high efficiency coding becomes possible to favorably reduce an amount of information to be sent to the decoding device if the stationary and the moving image blocks could separately be processed. From this fact, it is readily understood that the stationary and the moving image blocks may be judged as a less significant block and a significant block.
However, less significant coded data signals may be wrongly transmitted to the decoding device when the transform errors are accumulated due to the finite precision of the code transform and the inverse code transform. Such wrong transmission of the less significant coded data signals results in an increase of an amount of information to be transmitted and brings about a reduction of data transmission efficiency.